The present invention relates generally to the field of monovinylarene-conjugated diene block copolymers. More particularly, it concerns blends of such copolymers with other monovinylarene-containing polymers.
Thermally-formed packaging articles include categories that may be referred to herein as thermoformed packaging articles and shrink labels. Thermoformed packaging articles, for example, so-called clamshell packages and blister packs, are well known for packaging relatively small, relatively light articles, especially those for which it is desired to allow a consumer to visually inspect the article prior to purchase. Examples of packaged products include, but are not limited to, portable electronic devices (such as personal music players, telephone accessories, handheld electronic games, computer accessories, and game console accessories, among others), prepackaged foods (such as pudding cups, among others), and medical supplies, among others. To allow visual inspection and consumer handling of thermoformed packaging articles, such articles are generally made from materials having both high clarity and high toughness.
Shrink labels are known for use in labeling preformed packages or sealing the closures of preformed packages. Examples of such uses of shrink labels include, but are not limited to, labels on contoured bottles of beverages and containers of prepackaged foods and tamper-evident seals over the caps of pharmaceutical bottles, among others.
Two materials frequently used in thermally-formed packages are polyvinylchloride (PVC) and glycol-modified polyethylene terephthalate glycol (PETG). PETG is generally considered to have a relatively low glass transition temperature and a relatively high shrinkage upon exposure to heat, and as a result has been frequently used in shrink film applications. However, PETG is relatively expensive and has low melt strength, low heat deflection temperature, a tendency to absorb moisture, and relatively high specific gravity. Polyvinyl chloride (PVC) is relatively inexpensive, and has been used in thermally-formed packaging applications, but its poorer shrink performance (in shrink film applications), relatively high specific gravity, formation of corrosive and hazardous gases during extrusion, and a negative public perception of both several plasticizers often used to soften PVC and several of its incineration products render it less desirable.
Styrenic polymers, including styrene-butadiene copolymers, do not have these disadvantages. However, it remains challenging to produce thermally-formed packaging articles containing styrene-butadiene copolymers which have clarity, toughness, and shrink properties competitive with PVC or PETG.